261 minus 235 equals 26 cubic inches more air through the carburetor every two turns of the engine. You've probably had to open the throttle (yes, that's the bottom butterfly) enough to expose the vacuum port to make it flow enough air to idle properly. Start by drilling a 1/8" hole beside the throttle shaft, so some air can flow through the throttle plate to keep you from having to crank the idle speed screw open so far. If that's not enough, increase the size of the hole a few thousandths at a time until the engine will idle with the throttle plate almost closed. This assures good manifold vacuum at idle, and also prevents the distributor from over-advancing.
One good way to estimate the timing is to drive up a moderate grade, at about half throttle in high gear, and see if the engine detonates. That's a noise like marbles rattling around in a tin can that goes away if you ease up on the gas. If that happens, retard the timing a couple of degrees and try again. The idea is to get the most timing advance without getting any detonation. At that point, you'll get the best performance, and gas mileage, with no engine damage from too-early timing.
In the early 50's, when my family was racing alcohol-fueled flathead Ford engines in 1934 3-window coupes, we did some experiments on timing. With the car running on a chassis dynamometer, we would advance the timing at full throttle until the maximum horsepower was developed, and record the HP reading. Then the timing would be advanced one more degree, and the power loss recorded. Back to max HP by retarding ther timing to its former number, and start retarding it and recording power loss. It was necessary to retard the timing FIVE DEGREES to drop as much power as one degree too much advance! When in doubt, don't push the limits of timing advance! It's easy to go too far!
Jerry
